Generally, in an optical device such as a liquid crystal display element, an organic EL (electro luminescent) element, and a solid-state image sensor, in order to prevent the element surface from being exposed to a solvent or heat during the production process thereof, a protective film is provided. For the protective film, there are required not only having high adhesion with a substrate to be protected and having high solvent resistance, but also being excellent in performance such as heat resistance.
In addition, in the case where such a protective film is used as a protective film of a color filter used in a color liquid crystal display device or a solid-state image sensor, in order to maintain the transmittance of light transmitting a color filter, the protective film is required to have high transparency.
In recent years, cost reduction and weight reduction by introducing a retardation material into a cell of a liquid crystal display have been studied and as such a retardation material, there is generally used a material prepared by applying a polymerizable liquid crystal solution to a substrate to align the solution and by photo-curing the resultant coating. For aligning the retardation material, an underlayer film thereof is necessary to be a material having alignment properties after the underlayer film has been subjected to a rubbing process or a polarized UV irradiation. Therefore, the retardation material is formed on a liquid crystal alignment layer film formed on an overcoating of a color filter. (see FIG. 1(a)). If a film serving as both this liquid crystal alignment layer and the overcoating of the color filter (see FIG. 1(b)) can be formed, large advantages such as the reduction of the cost and the reduction of the number of the processes can be obtained, so that a material capable of serving as both the liquid crystal alignment layer and the overcoating is strongly desired.
Generally, for the overcoating of the color filter, an acrylic resin having high transparency is used. Then, for dissolving the acrylic resin, there are widely used a glycol-based solvent such as propylene glycol monomethyl ether and propylene glycol monomethyl ether acetate; an ester solvent such as ethyl lactate and butyl lactate; and a ketone solvent such as cyclohexanone and methyl amyl ketone from the viewpoint of handling properties and coatability. With respect to the acrylic resin, there is taken a technique for enhancing heat resistance and solvent resistance of the acrylic resin by thermo-curing or photo-curing the acrylic resin (Patent Documents 1 and 2). However, although a conventional thermo-curable or photo-curable acrylic resin has appropriate transparency or solvent resistance, even when an overcoating formed from such an acrylic resin is subjected to a rubbing process or a polarized UV irradiation, satisfactory alignment properties cannot be obtained.
For the liquid crystal alignment layer, a material containing a solvent-soluble polyimide or polyamic acid is usually used. It is disclosed that such a material is completely imidated by being postbaked, so that solvent resistance becomes imparted to the material and by being subjected to a rubbing process, the material becomes a material exhibiting satisfactory alignment properties (Patent Document 3).
It is disclosed that by irradiating an acrylic resin having in a side chain thereof, a photodimerized moiety such as a cinnamoyl group and a calcone group with polarized UV, satisfactory liquid crystal alignment properties can be exhibited (Patent Document 4).
Furthermore, there is disclosed a liquid crystal aligning agent containing a polymerizable component having a structure to be crosslinked by being heated (Patent Document 5).